Receiver crystal



Feb. 14, 1956 c. P. DOMENICHINI ETAL 2,735,071

DUPLEXING SYSTEMS Filed Aug. 31 1951 TO RECEIVER CRYSML CONVERTER LOW 37 23 PASS FILTER TO OSCILLATOR 7U ANTENNA m 2 V F/33 LOW FREa/EA/cm/37 245s FILTER lNl/EN7D/2S CARLO P. DOMENICHINI HARRY J. THOMAS, J/2.

A TTIORNEY DUPLEXING SYSTEMS Carlo P. Domenichini, Malden, and Harry J. Thomas, Jr., Waitham, Mass, assignors to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application August 31, 1951, Serial No. 244,678

3 Claims. (Cl. 333-13) This invention relates to signal switching devices, and more particularly to duplexing systems for use in pulse echo radar equipment.

It is well known that, in pulse radar equipment wherein the transmitter and receiver use the same antenna, a duplexing system is required to isolate the receiver from the transmitter during the transmitted pulse period. Where very sensitive receiver mixer devices are used, such as crystals, it is imperative that the isolation produced by the duplexer be extremely high, since relatively small amounts of power will destroy a crystal.

When the peak pulse power of the transmitter exceeds five hundred kilowatts, a single-tuned resonant cavity acting as a duplexer between the receiver and the line connecting the transmitter with the antenna will not produce suflicient isolation. At the higher microwave frequencies, for example, three thousand megacycles and above, additional isolation has previously been achieved by positioning an iris covered with a dielectric sheet in, a Wave guide structure connecting the tuned resonant cavity to the main transmission line. The high power pulse from the transmitter causes a discharge between the sides of the iris along the surface of the dielectric, thereby producing additional isolation of the receiver from the main transmission line over what could be accomplished by a single tuned resonator alone.

This device, commonly known as a pretuned resonant iris, is satisfactory at the higher microwave frequencies. However, at lower microwave frequencies, for example, on the order of one thousand megacycles, the wave guide size required in which an iris could be placed and the size of the iris itself would be quite large. Such a device is unfeasible, due to the size and bulk, and, in addition, trouble would probably be encountered with stresses in the large dielectric covering the iris, due to heating by the discharge.

This invention discloses a duplexing system which will provide sufficient isolation between the main transmission line and the receiver at peak powers considerably in excess of five hundred kilowatts and at frequencies on the order of two thousand megacycles or lower.

Briefly, this invention comprises two cavities critically coupled together, for example, by a transmission line substantially a quarter wave length long at the operating frequency of the system, said cavities containing gaps adapted to be broken down by the transmitted pulse. The first cavity is coupled by means of an iris to the main transmission line which comprises a coaxial line, and the second cavity is coupled through a coaxial transmission line and a low frequency pass absorption harmonic filter, whose cut-off frequency is adjusted to a frequency intermediate the fundamental and second harmonic of the operating frequency of the system, to the radar receiver.

in order to insure breakdown of the gaps in both cavities by a transmitted pulse, the loaded Q of the secondcavity is adjusted by adjusting the coupling between the cavity and the coaxial lines until it is substantially higher nited States Patent C ice than the loaded Q of the first cavity which is adjusted by adjusting the coupling to the main transmission line and the transmission line connecting the two cavities. In addition, the direct coupling, from the main transmission line through the first cavity to the coaxial line coupling the two cavities together, is made relatively large. This is accomplished by positioning the means, which couple said lines to the first cavity, relatively close together in the first cavity. As a result a substantial portion of the transmitted pulse is fed directly through said coupling means to the second cavity.

Other and further objects and advantages of this invention will become apparent as the description thereof progresses, reference being had to the accompanying drawing, wherein:

Fig. 1 illustrates a longitudinal, cross-sectional view of a duplexing system embodying this invention; and

Fig. 2 illustrates a transverse, cross-sectional view of the device shown in Fig. 1, taken along line 22 of Fig. 2.

Referring now to Figs. 1 and 2, there is shown a main transmission line comprising a coaxial line 10 having an outer conductor 11 and an inner conductor 12. Transmission line 10 has one end thereof connected to a radar transmitter such as, for example, a magnetron, and the other endthereof is connected to an antenna. The transmitter and antenna are neither shown nor described, but may be of any desired type well known in the art.

Transmission line 10 is coupled to a first tuned resonant cavity 13 by means of an iris 14 comprising a rectangular opening cut in outer wall 11. Cavity 13 comprises an outer cylinder 15 having an opening therein engaging iris 14. Cylinder 15 is covered by end plates 16 and 17. Extending into cylinder 15 from plate 17 is an inner cylinder 18 positioned coaxial with cylinder 15 and-extending for a distance on the order of two-thirds of the length of cylinder 15.

Positioned between the unattached end of cylinder 18 and end plate 16 is a duplexer tube 19. Duplexer tube 19 may be of any desired type, and is shown here diagrammatically as comprising a dielectric cylinder 20 whose ends are closed by frusto conical metallic gap members 21. and 22, respectively. It is to be clearly understood that duplexer 19 is shown here by way of illustration only and any desired duplexing tube or device could be used. For example, it has been found that the standard duplexer tube 1B27 having the conventional keep-alive electrode and tuning mechanism works extremely well with this system.

Inner cylinder 18 and outer cylinder 15 form a coaxial transmission line which is made effectively a quarter wave length long by the addition of end plates 16 and 17 and duplexer tube 19. Since one end of the transmission line is shorted by plate 17 and the other end thereof is open, due to the gap in the duplexer tube 19, the coaxial line behaves as a resonator with substantially the highest electrostatic fields appearing directly across the gap in duplexer 19.

Cavity 13 is coupled to a second cavity 23 having an outer cylinder 24 similar to cylinder 15, an end plate 25 similar to. end plate 17, an end plate 26 similar to end plate 16, an inner cylinder 27 similar to cylinder 18, and a duplexer tube 23 similar to duplexer tube 19. The coupling between the cavities 13 and 23 is accomplished by a coaxial line comprising an outer conductor 29 whose ends are connected to apertures in end plates 17 and 25, respectively. An inner conductor 30 is positioned inside conductor 29 coaxial therewith and has one end connected to one end of a loop 31' extending into cavity 13, the other end of loop 31 being attached to plate 17. The other end of inner conductor 3i -is connected to one end of a loop 32 extending into cavity 23, the other end of loop 32 being connected to plate 25.

Cavity 23 is coupled to an output coaxial line 33 comprising an outer conductor 34, one end of which covers an aperture in end plate 25. Line 33 has an inner conductor 35, one end of which is connected to one end of a loop 36 extending into cavity 23, the other end of loop 36 being connected to end plate 25. Line 33 passes through a low frequency pass harmonic filter 37 which may be, for example, of the type disclosed in copending application of John Reed, entitled Low Pass Filter, Serial No. 243,531, filed August 24, 1951.

After passing through filter 37, line 33 feeds a radar receiver of any desired standard type, not shown. This system is particularly useful when used with radar receivers having a crystal converter fed by line 33. The cut-off frequency of filter 37 is adjusted to a frequency above the fundamental resonant frequency of cavities 13 and 23 when duplexers 19 and 28 are not discharging, and below the second harmonic of said frequency.

By the use of high frequency filter 37, harmonics of the transmitter frequency which are produced by the pulsing action of the transmitter and which are not sufiiciently isolated from the receiver by the duplexing cavities 13 and 23 will be absorbed by the filter 37, and hence prevented from reaching the receiver crystal.

By making the length of coaxial line comprising conductors 29 and 30 substantially a quarter wave length long at the operating frequency of the system, maximum isolation is produced by the duplexing cavities 13 and 23 when the tubes 19 and 28 are fired. This results from the fact that, when the cavities are fired, they represent substantially short circuits at the operating frequency of the system, and hence are badly impedance mismatched to the coaxial line comprising conductors 29 and 30. It is to be clearly understood that the length of the coaxial line could be any odd number of quarter wave lengths long at the operating frequency of the system.

In order to insure that tube 28 in cavity 23 will always fire when a transmitted pulse is passed down line 10, the loaded Q of cavity 23 is made substantially greater than the loaded Q of cavity 13, and, as a result, tube 28 will fire before tube 19. When tube 28 fires, cavity 23 becomes effectively a short circuit at the operating frequency of the system. This reflects an impedance through the coaxial line coupling cavities 13 and 23 together such that cavity 13, being spaced substantially a quarter wave length from the short circuit presented by cavity 23, sees a substantially open circuit. As a result, the loaded Q of cavity 13 increases when tube 28 in cavity 23 fires, thereby increasing the electrostatic field across the gap in tube 19 and insuring the firing thereof.

By way of example, the system works well when the loaded Q of cavity 23 is on the order of two hundred and fifty and the loaded Q of cavity 13 is on the order of fifty when the tubes are not firing. The Qs of the cavities are adjusted by adjusting the degree of coupling between the cavities and the coaxial lines connected to the cavities. For example, loops 32 and 36 are made relatively small, thereby loosely coupling cavity 23 to these lines. As a result, cavity 23 is only lightly loaded by the characteristic impedances of the lines 29 and 33, and hence has a relatively high Q.

On the other hand, loop 31 is made relatively large, thereby coupling cavity 13 relatively tightly to the coaxial line, and hence loading it relatively heavily. In addition, iris 14 is made relatively large, thereby heavily loading cavity 13 by transmission line 10. It is to be clearly understood that the particular method of loading the cavities to produce the different Qs is shown here by way of example only, and that any desired method of cavity loading may be used.

This completes the description of the particular embodiment of the invention described herein. However, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of the invention. For example, other transmission devices, such as parallel wire lines and wave guides, may be used in place of the coaxial cables illustrated herein, and other types and shapes of cavities and duplexing structures may be used. Accordingly, it is desired that this invention be not limited by the particular details illustrated herein, except as defined by the appended claims.

What is claimed is:

1. A signal wave transmission network comprising a first toroidal cavity containing a gap adapted to discharge electrically under the influence of a predetermined electric field, a second toroidal cavity containing an electrical discharge gap, said cavities being substantially resonant to the operating frequency of said network, first means for reactively coupling said first cavity to a transmission line, second means for reactively coupling said first cavity to said second cavity, said first and second coupling means being positioned in close physical relationship in said first cavity such that a substantial portion of the energy of a transmitted pulse is fed directly through said coupling means to said second cavity whereby the gap in said second cavity is discharged before the gap in said first cavity.

2. A signal wave transmission network comprising a first toroidal cavity containing a gap adapted to discharge electrically under the influence of a predetermined electric field, a second toroidal cavity containing an electrical discharge gap, said cavities being physically spaced and substantially resonant to the operating frequency of said network, first means for reactively coupling said first cavity to a transmission line, second means for reactively coupling said first cavity to said second cavity, said second means including a network substantially an odd number of quarter wave lengths long, said first and second coupling means being positioned in close physical relationship such that a substantial portion of the energy of a transmitted pulse is fed directly through said coupling means to said second cavity whereby the gap in said second cavity is discharged before the gap in said first cavity.

3. A signal wave transmission network comprising a first toroidal cavity containing a gap adapted to discharge electrically under the influence of a predetermined electric field, a second toroidal cavity containing an electrical discharge gap, said cavities being physically spaced and substantially resonant to the operating frequency of said network, first means for reactively coupling said first cavity to a transmission line, second means for reactively coupling said first cavity to said second cavity, said second means including a network substantially an odd number of quarter wave lengths long, said first and second coupling means being positioned in close physical relationship such that a substantial portion of the energy of a transmitted pulse is fed directly through said coupling means to said second cavity whereby the gap in said second cavity is discharged before the gap in said first cavity, and an output coupled to said second cavity comprising a low frequency pass absorption filter, said filter having a cutolf frequency slightly above the resonant frequency of said cavities.

References Cited in the file of this patent UNITED STATES PATENTS 2,422,190 Fiske June 17, 1947 2,478,332 Smullin et al. Aug. 9, 1949 2,522,861 Cork Sept. 19, 1950 2,567,701 Fiske Sept. 11, 1951 2,582,205 Longacre Jan. 8, 1952 2,656,534 Jackson Oct. 20, 1953 OTHER REFERENCES Radio Engineering, F. E. Terman, third edition, McGraw-Hill Book Co., Inc. New York, 1947, pages 

